In a reaction injection molding (RIM) dispensing system such as those used to mix and dispense liquid chemical components to create a urethane foam, the liquid components are normally impingement mixed in a mixhead by injecting them through opposed injector ports into a mixing chamber in the mixhead at either a relatively low or high pressure. The mixed chemicals are dispensed therefrom as a shot by a dispensing wand that is connected to the mixing chamber at the mixhead. The dispensing wand may be a simple pipe or tube of uniform internal diameter and various lengths for dispensing the material into a mold cavity or it may have a spray tip for dispensing the material in a spray pattern onto a mold surface or an object requiring a foam coating. In the low pressure type systems, the chemicals are injected into the mixing chamber with pressures of about 50-200 psi and a mechanical aftermixer such as a spiral element is normally used between the injector ports and the outlet end of the dispensing wand to obtain thorough mixing. In the high pressure type systems, the chemicals are injected into the mixing chamber with pressures as high as 2000 psi or more to enhance their impingement mixing so that an aftermixer is not normally required.
Following each dispensing or shot of the mixed chemicals, a solvent is normally flushed through the mixhead to clean away any retained mixed chemicals in the low pressure systems but not always in the high pressure systems as the mixed chemicals are dispensed very quickly from the mixhead provided the dispensing wand is short and not a spray wand having a flow restricting spray tip or nozzle. In the high pressure mixheads, there is commonly employed a hydraulically operated injector valve/clean out plunger in the mixing chamber that opens the injector ports to admit the highly pressurized liquid chemicals into the mixing chamber and then closes and wipes these ports and the mixing chamber in both a valving and cleaning action. Depending on the particular dispensing wand being used with the high pressure mixhead, the residence or dwell time of the mixed chemicals in the mixhead including the dispensing wand may be so great as to require solvent flushing. For example, with long dispensing wands, the resulting long dwell time of the fast reacting urethane polymer in the mixing chamber and/or the dispensing wand can result in a substantial increase in the viscosity of the polymer passing through so that it can not be cleaned well following a shot and a solvent flush must then be employed. But even then, some of the reacting polymer can be left in the mixing chamber and/or dispensing wand and can build up over time with repeated use to the point where it clogs the dispensing wand and causes the injector valve/clean out plunger in a high pressure mixhead to stick. The tendency for such clogging is particularly acute in the case of a spray wand because of the spray tip at its end that back pressures the wand and mixing chamber. When the dispensing wand becomes clogged, maintenance time is then required to disassemble and clean the affected parts. And when the injector/clean out plunger sticks, the resulting stress from its hydraulically forced movement can be so high as to cause its breakage. There is a particular tendency for the reacting polymer to build up at the conventional injector valve/clean out plunger causing it to stick because it blocks the injector ports from the solvent and produces a dead space in the mixing chamber where the solvent is not able to thoroughly clean and flush the plunger of retained chemicals.